1. Technical Field of the Invention
The present invention relates generally to a valve actuating device equipped with an electrically operated actuator and a fuel injector for internal combustion engines equipped with such a valve actuating device.
2. Background Art
Hydraulic fuel injectors equipped with a piezoelectric valve actuator are used in internal combustion diesel engines of automotive vehicles. Such a fuel injector includes a large-diameter piston moved by the expansion and contraction of the piezoelectric valve actuator, a pressure chamber filled with hydraulic fluid, and a small-diameter piston which are arranged in alignment with each other. The movement of the large-diameter piston causes the hydraulic fluid in the pressure chamber to change in pressure, which moves the small-diameter piston. The small-diameter piston then actuates a control valve.
When it is required to emit a fuel spray, the piezoelectric valve actuator is energized so that it expands to increase the hydraulic pressure in the pressure chamber through the large-diameter piston. This causes the expansion of the piezoelectric valve actuator to be amplified hydraulically and transmitted to the small-diameter piston. The small-diameter piston then moves downward and opens the control valve. When the control valve is opened, it will cause the pressure in a back pressure chamber to drop, thereby lifting up a nozzle needle to initiate fuel injection. Contracting the piezoelectric valve actuator will cause the small-diameter piston to move upward, thereby closing the control valve to terminate the fuel injection.
There is known the above type of fuel injector which has disposed therein a hydraulic mechanism designed to supply working fluid to the pressure chamber through a check valve in order to compensate for a leakage of working fluid from the pressure chamber. For example, U.S. Pat. No. 5,779,149 to Hayes, Jr. teaches a fuel injector which has formed therein a fluid passage serving to direct the fuel leaking from a nozzle needle to a pressure chamber through a check valve made up of a ball valve and a coil spring. U.S. Pat. No. 6,155,532 (corresponding to Japanese Patent First Publication No. 11-166653) teaches a fuel injector which has a refill valve disposed in a radial direction of a pressure chamber for compensating for a leakage of fuel from the pressure chamber. The refill valve is, like the above structure, made up of a ball valve and a coil spring.
The above structures, however, have three drawbacks as discussed below.
(1) The pressure chamber being filled with the working fluid after assembly of the fuel injector, air may be left in the pressure chamber, thus resulting in instability of operation of the fuel injector. (2) The small-diameter piston falls downward by the gravity while the fuel injector is at rest for a long period of time, so that an amount of working fluid equivalent to a change in volume of the pressure chamber is supplied to the pressure chamber through the check valve, thereby making it difficult to lift up the small-diameter piston, which disenables a subsequent operation of the fuel injector. (3) If power supply to the piezoelectric valve actuator is cut undesirably during expansion of the piezoelectric valve actuator, it becomes impossible for the piezoelectric valve actuator to contract, thus resulting in the pressure in the pressure chamber being kept at higher levels, which causes the fuel to continue to be sprayed from the fuel injector. Further improvement of controllability and safety of fuel injectors is, therefore, sought.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of a valve actuating device which assures higher controllability and safety in operation and a fuel injector equipped with such a valve actuating device.
According to one aspect of the invention, there is provided a valve actuating device which may be used in a fuel injector for automotive internal combustion engines. The valve actuating device comprises: (a) an actuator; (b) a first piston displaced by the actuator; (c) a second piston operating a valve, the second piston being smaller in diameter than the first piston; (d) a displacement amplifying chamber provided between the first piston and the second piston, the displacement amplifying chamber being filled with working fluid to amplify and transmit displacement of the first piston to the second piston; and (e) a drain passage communicating with the displacement amplifying chamber through a pinhole for draining the working fluid within the displacement amplifying chamber.
In the preferred mode of the invention, the diameter of the pinhole is within 0.02 to 0.5 mm.
A check valve is disposed between the displacement amplifying chamber and the drain passage which allows the working fluid to flow only from the drain passage to the displacement amplifying chamber. The check valve includes a flat valve in which the pinhole is formed.
The first piston has formed therein a passage leading to the drain passage. The pinhole is provided between the passage and the displacement amplifying chamber.
The first piston has a length in which the passage extends longitudinally and has an opening formed in a first end of the length exposed to the displacement amplifying chamber. The flat valve of the check valve is disposed on the opening of the passage to allow the working fluid to flow only from the drain passage to the displacement amplifying chamber through the passage. The pinhole is formed in the flat valve of the check valve.
An oil sump is formed on a side of a second end of the first piston opposite the first end and establishes fluid communication between the drain passage and the passage.
A spring chamber is formed on the side of the first end of the first piston in which a spring is disposed to urge the actuator away from the displacement amplifying chamber. The spring chamber defines the oil sump.
According to another aspect of the invention, there is provided a fuel injector which may be employed in automotive internal combustion engines. The fuel injector comprises: (a) an injector body; (b) a fuel inlet passage formed in the injector body; (c) an actuator; (d) a first piston displaced by the actuator; (e) a second piston smaller in diameter than the first piston, the second piston operating a valve for spraying fuel supplied from the fuel inlet passage from a spray hole; (f) a displacement amplifying chamber formed between the first piston and the second piston within the injector body, the displacement amplifying chamber being filled with working fluid to amplify and transmit displacement of the first piston to the second piston; and (g) a drain passage formed in the injector body which communicates with the displacement amplifying chamber through a pinhole for draining the working fluid within the displacement amplifying chamber.
In the preferred mode of the invention, the displacement amplifying chamber is filled with the working fluid at a factory.
The working fluid is injected into the displacement amplifying chamber at the factory after the displacement amplifying chamber is evacuated.
The injector body is sealed to avoid leaking of the working fluid in the displacement amplifying chamber out of the injector body.